CN113067601B

CN113067601B - System and method for improving initial synchronization performance of direct sequence spread system and direct sequence spread power meter reading system

Info

Publication number: CN113067601B
Application number: CN202110320016.0A
Authority: CN
Inventors: 段红光; 敬天成; 应俊; 郑建宏; 林毅
Original assignee: Chongqing University of Post and Telecommunications
Current assignee: Chongqing University of Post and Telecommunications
Priority date: 2021-03-25
Filing date: 2021-03-25
Publication date: 2022-06-10
Anticipated expiration: 2041-03-25
Also published as: CN113067601A

Abstract

The invention relates to a system and a method for improving initial synchronization performance of a direct sequence spread spectrum system and application of the system in a direct sequence spread spectrum electric meter reading system, belonging to the field of communication of the Internet of things, wherein the system comprises a receiving baseband signal unit, a correlation calculation unit, a leading segment data correlation peak value search unit and a frequency deviation calculation unit, wherein a local one-bit leading chip generates a segment chip unit, and the segment chip generates a local baseband signal unit; receiving data from a radio frequency unit, completing automatic gain control and analog-to-digital conversion, and forming digital baseband data; generating a local preamble chip sequence according to the requirements of the spread spectrum code and the spread spectrum factor, and dividing the chip sequence into a plurality of sections of chip sequences; modulating the segmented chip sequence to generate corresponding baseband data; performing correlation calculation on the local leading segmented chip baseband data and the received baseband signal, and searching to obtain the starting position and the ending position of a leading bit; and carrying out frequency estimation by using the related likelihood value to obtain the frequency deviation.

Description

System and method for improving initial synchronization performance of direct sequence spread spectrum system and direct sequence spread spectrum electric meter reading system

Technical Field

The invention belongs to the field of communication of the Internet of things, and relates to a system and a method for improving initial synchronization performance of a direct sequence spread spectrum system and application of the system and the method in a direct sequence spread spectrum electric meter reading system.

Background

Direct Sequence Spread Spectrum (DSSS) techniques, referred to as Direct Sequence Spread Spectrum (DSSS) techniques, encode a bit of data into a Sequence of bits, called a "chip". For example, data bit "0" is encoded with chip "00100111000", data bit "1" is encoded with chip "11011000111", and data string "010" is encoded with "00100111000" or "11011000111". "00100111000".

The direct sequence spread spectrum has the advantages of strong anti-interference capability, strong anti-multipath interference capability, strong anti-interception capability, capability of working at the same frequency, convenience for realizing multiple access communication and the like, the direct sequence spread spectrum technology is widely applied to military communication and confidential industry, is even popularized to some civil high-end products, such as signal base stations, wireless televisions, cellular phones, wireless baby monitors and the like, and is a reliable and safe industrial application scheme.

A scenario is presented herein in which direct sequence spread spectrum is used in power meter reading. The electric power meter reading system adopts a frame burst mode for communication, and the electric power meter reading burst frame structure comprises three parts, namely a synchronization head (short for SHR), a physical layer frame head (short for PHR) and a physical layer service data unit (short for PSDU), wherein the SHR is mainly used for completing burst frame data block searching, frequency synchronization, timing synchronization and frame synchronization; the PHR provides a signaling for resolving the PSDU; the PSDU carries protocol signaling and service data content of electric power meter reading.

In the frame burst of the power meter reading, the SHR is divided into two parts, namely a Preamble (short for Preamble) and a Start of Frame Delimiter (SFD), the Preamble is composed of a plurality of 0 bits, and the length of the 0 bits of the Preamble is recommended to be different under different PSDU transmission rates. The Preamble provides the receiving end to perform automatic gain control, frame burst detection, and preliminary adjustment of the frequency and timing of burst data blocks.

The frame start delimiter SFD is composed of fixed bit sequence, Preamble provides automatic gain control, frame burst detection, and frequency and timing preliminary adjustment of burst data block at receiving end. The start of frame delimiter SFD indicates the Preamble end position and the physical layer frame header PHR start position.

Because the electric meter reading is transmitted by adopting a direct spread spectrum mode, the Preamble and the SFD data of the SHR also need to be transmitted after direct spread spectrum, in the electric meter reading application scene, the Preamble adopts a fixed spread spectrum factor 256 to carry out direct spread spectrum transmission, and the SFD adopts several fixed spread spectrum factors to carry out transmission.

From theory, in the direct spreading sequence, the larger the spreading factor, the better the performance, so 256 is adopted as the spreading factor of Preamble in the frame burst data structure.

However, in practical engineering, after receiving frame burst data, the receiving end demodulates in a time domain correlation mode, and since the Preamble spreading factor increases and there is an initial frequency difference between the transmitting end and the receiving end, this directly affects the receiving end correlation result. When a frequency deviation scene of 200Hz exists in the base bands of the receiving end and the transmitting end, the correlation peak of the receiving end is still obvious, but interference exists. When a frequency deviation scene of 400Hz exists in the base bands at the transmitting end and the receiving end, a plurality of correlation peaks exist at the receiving end, and an effective correlation peak value cannot be clearly judged. However, in practical engineering, the baseband frequency deviation at the transmitting end and the receiving end is generally over 1000Hz, and the limit condition can reach 2000Hz, so when the spreading factor adopts 256, the 256 chips are directly adopted for correlation processing, the effect is definitely not ideal, and a high requirement is put forward on a hardware clock crystal oscillator.

Disclosure of Invention

In view of the above, the present invention provides a method for improving synchronization accuracy of direct spread spectrum communication technology.

In order to achieve the purpose, the invention provides the following technical scheme:

on one hand, the invention provides a system for improving the initial synchronization performance of a direct sequence spread spectrum system, which comprises a baseband signal receiving unit, a correlation calculation unit, a preamble segment data correlation peak value searching unit, a frequency deviation calculation unit, a segment chip generating unit of a local one-bit preamble chip, and a local baseband signal generating unit of the segment chip;

the receiving baseband signal unit receives data from the radio frequency unit, completes automatic gain control, and performs analog-to-digital conversion to form digital baseband data;

the local one-bit preamble chip generation segment chip unit generates a local preamble chip sequence according to the requirements of a spread spectrum code and a spread spectrum factor in a direct spread spectrum system, and divides the chip sequence into a plurality of segments of chip sequences according to the requirements;

the chip segmentation generation local baseband signal unit modulates the segmented chip sequence to generate corresponding baseband data;

the correlation calculation unit performs correlation calculation on the local leading subsection chip baseband data and the received baseband signal, and performs correlation calculation once when receiving one chip symbol data;

the leading segment data correlation peak value searching unit is used for searching the calculation result of the correlation calculation unit to obtain the starting position and the ending position of the leading bit;

and the frequency deviation calculating unit carries out frequency estimation by using the correlation likelihood value obtained by the correlation calculating unit to obtain the frequency deviation.

Further, the frequency deviation calculation unit calculates a phase difference between two adjacent correlation peaks according to the determined correlation values, calculates a time interval between the two adjacent correlation peaks, calculates a frequency deviation of a baseband between the receiving end and the transmitting end, and adjusts the baseband frequency.

On the other hand, the invention provides a method for improving the initial synchronization performance of a direct sequence spread spectrum system, which comprises the following steps:

step 1: the receiving end locally generates a chip sequence of a leading bit according to the spreading factor and the spreading code, and if the leading spreading factor is N, N chip sequence data are generated by leading data of one bit;

step 2: dividing the generated one-bit preamble chip sequence into K segments of chip data, namely segmentation chip sequences, recording the length of each segmentation chip data as S, wherein S meets the relation of S ═ N/K, and then modulating the segmentation chip sequences to form local preamble baseband signals; the receiving end samples the received baseband signal by adopting m times of chip rate, and then the m times of up-sampling processing is carried out on the local segmented chip sequence modulation signal, namely, each data in the local segmented chip sequence modulation signal is repeated for m times;

and step 3: receiving frame burst data to form a baseband data stream, wherein the baseband data stream is up-sampled by m times of a baseband signal;

and 4, step 4: respectively carrying out correlation calculation on the received baseband data stream and the local segmented chip sequence modulation signal; when receiving a sampling data, the receiving end performs correlation calculation with a local segmented chip sequence modulation signal once, and then obtains K correlation peak values each time;

and 5: the receiving end searches the correlation peak, if K obvious correlation peak values exist, the receiving end indicates that the receiving end has searched a leading baseband signal of a bit 0 in the frame burst;

step 6: the receiving end determines the starting position and the ending position of the leading bit according to the position of the correlation peak, then uses the correlation value of the correlation peak to calculate the phase difference between two adjacent correlation peak values, and uses the position difference between the two correlation peaks to calculate the time length between the two correlation peaks, and calculates the baseband signal frequency difference between the receiving end and the transmitting end;

and 7: and the receiving end adjusts the frequency of the received baseband signal by using the baseband signal frequency difference calculated in the step 6.

On the other hand, the invention provides a system for improving the initial synchronization performance of a direct spread power meter reading system, wherein the direct spread power meter reading system adopts a frame burst mode for transmission, each burst data block consists of an SHR, a PHR and a PSDU, and the burst data block is modulated by adopting an Offset Quadrature Phase Shift Keying (OQPSK) modulation mode;

the power meter reading receiving preamble comprises a power meter reading signal receiving module, a local preamble chip data generating module, a local preamble baseband data generating module, a preamble correlation calculating module, a preamble section data correlation peak value searching module and a frequency synchronization calculating module;

the receiving end of the power meter reading signal receiving module carries out down-conversion processing on a signal received from the radio frequency front end, then carries out analog-to-digital conversion, and carries out up-sampling processing at the receiving end by adopting 8 times of chip rate to obtain two paths of I/Q baseband digital signals at 8 times of speed;

a local preamble chip data generation module, wherein the preamble adopts a fixed spreading factor 256, one bit of 0 is led in, the spreading is carried out to 256 chip sequences, and then the chip is segmented by adopting 16-segment or 8-segment modes to form two baseband chip segment data;

the local leading baseband data generating module performs OQPSK modulation on baseband chip segmented data locally at a receiving end to form two paths of data I and Q, performs 8 times of up-sampling on the segmented leading baseband data, and generates 16 groups of segmented leading baseband data of 16x8 under the condition of 16 segments; in the case of 8 segmentation, 8 groups of 32x8 of segmented preamble baseband data are generated;

the leading correlation calculation module is used for finishing correlation calculation of the received baseband digital signals and the segmented leading modulation baseband data, and performing correlation calculation once when a baseband digital signal sampling value is received to obtain 16 or 8 correlation values;

a preamble segment data correlation peak searching module, which performs modular calculation on 16 or 8 correlation values calculated by the preamble correlation module to obtain 16 or 8 correlation peak values, and if the 16 or 8 correlation peak values are greater than a certain threshold, determines that a receiving end searches for an effective preamble 0-bit baseband signal;

and the frequency synchronization calculation module firstly calculates the phase difference between two adjacent correlation peaks according to the determined 16 or 8 correlation values, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the frequency deviation of a baseband between the receiving end and the transmitting end, so that the frequency deviation is used for adjusting the baseband frequency by the power meter reading signal receiving module.

Further, the local segmentation principle includes the following:

firstly, carrying out correlation calculation by adopting received 256 multiplied by 8I/Q sampling data and subsequently received 256 multiplied by 8I/Q sampling data, and if an obvious correlation peak value exists, indicating that a receiving end searches for the starting and ending positions of chips of two leading 0 bits;

a receiving end receives 256 multiplied by 8I/Q data, 256 chips of I/Q baseband data are locally generated, the total length of the baseband data is 256 multiplied by 8, the baseband data is segmented into 16 sections of local baseband signals for the first time, and the length of each section of baseband signal is 16 multiplied by 8 and is used for coarse frequency adjustment;

the receiving end receives 256x 8I/Q data, 256 chips of I/Q baseband data are generated locally, the total length of the baseband data is 256x8, the second segment is divided into 8 segments of local baseband signals, and the length of each segment of local baseband signals is 32x8 and is used for fine frequency adjustment;

and (3) finishing frequency adjustment, wherein a lead code chip baseband signal with the length of 256x8 and a received signal are directly subjected to correlation calculation in the process, a correlation peak value is calculated, first fuzzy phase adjustment is carried out, and then the same method is adopted for second phase adjustment.

Further, the correlation calculation includes the following:

256 chips generated by the leading bit 0, and I/Q data generated by the two paths of chips are expressed in a complex manner, where the real part represents the I path and the imaginary part represents the Q path, and then the baseband data sent by the sending end is expressed as:

(a₁+b₁j),(a₂+b₂j),...,(a₂₅₆+b₂₅₆j)

the baseband signal sent by the sending end is expressed in a polar coordinate mode as follows:

the locally generated baseband signal is represented as:

and (3) correlation calculation, namely recording the correlation as conjugate correlation of the two signals, and then accumulating the correlation, wherein the correlation is expressed by a formula as follows:

further, the frequency deviation calculation method includes:

according to the xCORr value obtained by the correlation calculation, be is obtained by each calculation^jΨAbsolute value abs (be)^jΨ) That is, psi is a correlation peak, psi is a correlation phase, a correlation difference Δ psi between two adjacent correlation peaks is a phase deviation between two transmitting and receiving ends, and Δ t represents a time length of the two adjacent correlation peaks;

2πΔfΔt＝ΔΨ

namely, the frequency deviation Δ f of the baseband signals at the two transmitting and receiving ends is expressed as:

Δf＝ΔΨ/(2πΔt)。

after the frequency offset has been calculated, the phase ambiguity is also removed, i.e. multiplied by the received signal_e ^-jΨ。

Further, the frame leading search process of the electric power meter reading comprises the following steps:

frame search or frame header search in the electric meter reading system is completed at a receiving end, the receiving end continuously receives frame burst data from the transmitting end, the frame burst data passes through a radio frequency front end, down-conversion is carried out, and finally sampling is carried out at 8 times of a chip rate to obtain two paths of I/Q baseband signals;

the first local preamble chip segmentation method is as follows: leading a 0 bit locally, generating 256 chips according to the requirements of direct-spread power meter reading specifications, and dividing 256 chip sequence data into 16 segments, wherein each segment is a segmented chip sequence with the length of 16; according to the requirements of direct spread spectrum power meter reading specifications, OQPSK is adopted for modulation, and local baseband data of 16I/Q channels are formed; finally, carrying out 8 times of upsampling treatment on the 16I/Q local baseband data;

performing correlation processing by using 16 sections of segmented baseband data signals and received baseband signals, and calculating 16 sections of correlation calculation once each sampling value is received to obtain 16 correlation peak values;

preliminarily calculating the frequency deviation of the baseband signal by using the 16 calculated correlation values, and then performing frequency compensation on the received baseband signal by using the frequency deviation;

the second local preamble chip segmentation mode is as follows: locally leading one 0 bit to generate 256 chips, dividing 256 chip sequence data into 8 segments, and modulating each segment by using OQPSK (offset quadrature phase shift keying) to form local baseband data of 8I/Q two channels, wherein the segment chip sequence is 32 in length; finally, 8 times of upsampling processing is carried out on 8I/Q local baseband data;

and after the correlation calculation and the frequency adjustment are carried out on the local baseband signals in the first mode and the second mode in a segmenting mode, the correlation calculation is carried out by adopting the non-segmented baseband signals to obtain a correlation value, and the phase of the correlation value is used for carrying out phase compensation.

The invention has the beneficial effects that: in a direct sequence spread spectrum system, a frame burst mode is adopted for transmission, and because clock frequency deviation exists at the transmitting end and the receiving end, frequency deviation also exists on baseband signals at the transmitting end and the receiving end, if a longer spread spectrum sequence is adopted between the two ends to directly carry out correlation calculation on the received signals, the performance is poor, the frequency deviation cannot be used when reaching a certain range, and the receiving end cannot search the frame burst signal from the transmitting end.

In view of this, the invention segments the longer spreading chip sequence into the shorter chip sequence, and uses the shorter chip sequence to generate the local baseband signal and perform the correlation calculation with the received baseband signal, thereby suppressing the problem that the frequency deviation exists at the transmitting and receiving ends to influence the correlation performance. The invention ensures that the transceiver does not use a high-precision crystal oscillator, reduces the equipment cost and can still achieve good performance by a software method.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.

Drawings

For the purposes of promoting a better understanding of the objects, aspects and advantages of the invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the operation of a segmented chip;

FIG. 2 is a diagram of direct spreading sequence preamble segmentation;

FIG. 3 is a segmented chip workflow;

FIG. 4 is a flow chart of synchronous hair delivery of the direct expansion system;

FIG. 5 is a diagram of OQPSK modulation;

FIG. 6 is an initial synchronization schematic diagram of power meter reading;

FIG. 7 is a schematic diagram related to a local Preamble segment of the power meter reading;

FIG. 8 illustrates the principle of correlation calculation performed by the receiving end;

FIG. 9 is a power meter reading preamble synchronization process;

FIG. 10 is a diagram of the correlation results for (a) no segment, (b)16 segments and (c)8 segments;

fig. 11 is a 200Hz frequency deviation scenario for the transmit-receive baseband signal, wherein (a) is a correlation result diagram for no segment, (b) is a correlation result diagram for 16 segments, and (c) is a correlation result diagram for 8 segments;

fig. 12 is a 400Hz frequency deviation scenario of the transceiving baseband signal, wherein (a) is a correlation result diagram for no segment, (b) is a correlation result diagram for 16 segments, and (c) is a correlation result diagram for 8 segments;

fig. 13 is a 2500Hz frequency deviation scenario of the transmit-receive baseband signal, in which (a) a correlation result diagram is performed without segmentation, (b) a correlation result diagram is performed with 16 segments, and (c) a correlation result diagram is performed with 8 segments.

Detailed Description

The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.

Wherein the showings are for the purpose of illustrating the invention only and not for the purpose of limiting the same, and in which there is shown by way of illustration only and not in the drawings in which there is no intention to limit the invention thereto; to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.

The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "front", "rear", etc., based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not an indication or suggestion that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes, and are not to be construed as limiting the present invention, and the specific meaning of the terms may be understood by those skilled in the art according to specific situations.

Referring to fig. 1 to 13, according to the problems in the actual engineering, the present invention provides a method for parsing a preamble in a direct sequence spread spectrum system, which is based on the basic method of dividing a local one-bit preamble spreading chip into a plurality of chip sequences, referred to as segment chip sequences for short, using each segment chip sequence to form a baseband signal and a received baseband signal for correlation calculation, then using a phase difference between correlation peaks to calculate a frequency offset, and using the frequency offset to perform frequency compensation on a received signal.

The invention is composed of a baseband signal receiving unit, a correlation calculating unit, a leading segment data correlation peak value searching unit, a frequency deviation calculating unit, a local one-bit leading chip generation segment chip unit and a segment chip generation cost baseband signal unit. As shown in fig. 1.

The receiving baseband signal unit receives data from the radio frequency unit, completes automatic gain control, and performs analog-to-digital conversion to form digital baseband data; the local one-bit leading chip generation segment chip unit generates a local leading chip sequence according to the requirements of a spread spectrum code and a spread spectrum factor in a direct spread spectrum system, and divides the chip sequence into a plurality of segments of chip sequences according to the requirements; the chip segmentation generation local baseband signal unit carries out modulation processing on the chip sequences which are segmented, and generates corresponding baseband data; the correlation calculation unit completes the correlation calculation of the local leading subsection chip baseband data and the received baseband signal, and performs the correlation calculation once when receiving one chip symbol data; the leading segment data correlation peak value searching unit completes the calculation result of the correlation calculation unit and searches to obtain the starting position and the ending position of the leading bit; the frequency deviation calculating unit performs frequency estimation using the correlation likelihood value obtained by the correlation calculating unit to obtain the frequency deviation.

To further illustrate the method of segmenting a leading-bit data chip in the present invention, it is shown in fig. 2. Assuming that the preamble spreading factor is N in the direct sequence spread system, a preamble bit will form N chips of baseband data after direct sequence spread. The N chip data are divided into K segmented chip sequences, and each segmented chip sequence is composed of S chips in length. Wherein N, K and S satisfy the relationship of N-KxS.

Flow of the invention

Step 1: the receiving end locally generates a chip sequence of a leading bit according to the spreading factor and the spreading code, and if the leading spreading factor is N, the leading data of one bit generates N chip sequence data. As shown in step 1 of fig. 3.

Step 2: dividing the generated one-bit preamble chip sequence into K segments of chip data, which are also called segmented chip sequences, recording the length of each segmented chip data as S, wherein S satisfies the relation of S ═ N/K, and then modulating the segmented chip sequences to form local preamble baseband signals. If the receiving end samples the received baseband signal at m times of the chip rate, the local segmented chip sequence modulation signal is also subjected to m times of upsampling processing, that is, each data in the local segmented chip sequence modulation signal is subjected to m times of repetition. As shown in step 2 of figure 3.

And 3, step 3: frame burst data is received to form a baseband data stream, which is up-sampled by m times of the baseband signal. As shown in step 3 of figure 3.

And 4, step 4: and respectively carrying out correlation calculation on the received baseband data stream and the local segmented chip sequence modulation signal. When receiving a sampling data, the receiving end performs a correlation calculation with the local segmented chip sequence modulation signal once, and then obtains K correlation peak values each time. As shown in step 4 of figure 3.

And 5: and the receiving end searches the correlation peak, and if K obvious correlation peak values exist, the receiving end indicates that the receiving end has searched a preamble baseband signal of one bit 0 in the frame burst. As shown in step 5 of fig. 3.

Step 6: the receiving end determines the start and end positions of the leading bit according to the position of the correlation peak, then calculates the phase difference between two adjacent correlation peak values by using the correlation value of the correlation peak, calculates the time length between the two correlation peaks by using the position difference between the two correlation peaks, and calculates the frequency difference of the baseband signal between the receiving end and the transmitting end. As shown in step 6 of figure 3.

And 7: and the receiving end adjusts the frequency of the received baseband signal by using the baseband signal frequency difference calculated in the step 6. As shown in step 7 of figure 3.

In order to make clear that the invention is applied to actual engineering, the invention will be explained in a direct-spread power meter reading system. As introduced in the technical background, the direct spread power meter reading system uses a frame burst mode for transmission, and each burst data block is composed of an SHR, a PHR and a PSDU. The system adopts an Offset Quadrature Phase Shift Keying (OQPSK) modulation mode for modulation. The transmission link of the SHR part is shown in fig. 4.

The leading link of the direct expansion electric power meter reading system is the same as the electric power meter reading synchronous head link. After the synchronous data is prepared, the synchronous data is respectively subjected to differential coding, DSSS (direct sequence spread spectrum) spectrum spreading, framing and OQPSK (offset quadrature phase shift keying) modulation, and then is transmitted out through radio frequency. For the Preamble bit, the Preamble bit is composed of 0 bits, so that the differential coding has no effect on the Preamble bit. And the DSSS spread spectrum is spread by adopting a spread spectrum code of a spread spectrum factor 256 determined by the power meter reading system. After the spread code sequence is subjected to OQPSK modulation, the spread code sequence is submitted to a radio frequency front end to send out burst data containing a preamble frame. In the OQPSK modulation scheme, the mapping rule from the chip sequence to the I and Q channels is shown in fig. 5.

In this embodiment, the Preamble receiving and processing module is as shown in fig. 6, that is, an initial synchronization schematic diagram of power meter reading. The power meter reading receiving preamble is composed of a power meter reading signal receiving module, a local preamble chip data generating module, a local preamble baseband data generating module, a preamble correlation calculating module, a preamble section data correlation peak value searching module and a frequency synchronization calculating module.

In the power meter reading signal receiving module, a receiving end carries out down-conversion processing on a signal received from a radio frequency front end, then carries out analog-to-digital conversion (A/D change), and in order to improve demodulation performance, the receiving end adopts an 8-time chip rate up-sampling mode for processing, namely, frame burst data received from radio frequency is sampled by 8 times of a chip clock, so that the signal quality of the signal received by the receiving end is ensured, and finally, two paths of I/Q baseband digital signals at 8 times of speed are obtained.

And the local preamble chip data generation module adopts a spreading code defined by the bit 0 in the preamble to form a spreading sequence of the bit 0 according to the requirement of the power meter reading system. In this embodiment, the preamble employs a fixed spreading factor of 256. So leading by one bit 0, spread into a 256 chip sequence. The chip is then segmented, and in this embodiment, two segmentation methods, i.e., 16-segment or 8-segment, are used to form two baseband chip segment data.

The local preamble baseband data generating module performs OQPSK modulation on the baseband chip segment data locally at the receiving end to form two paths of I and Q data, which are also called segmented preamble baseband data, as shown in fig. 5. In the module, the power meter reading signal receiving module adopts 8 times of chip rate for sampling, so that the segmented leading baseband data is also subjected to 8 times of up-sampling. In the case of 16 segments, 16 sets of 16x8 of segmented preamble baseband data are generated. In the case of 8-segment, 8 groups of 32x8 of segmented preamble baseband data are generated.

And the preamble correlation calculation module is used for performing correlation calculation on the received baseband digital signals and the segmented preamble modulation baseband data, and performing correlation calculation once every time a baseband digital signal sampling value is received to obtain 16 or 8 correlation values.

And the preamble segment data correlation peak searching module is used for performing modular calculation on 16 or 8 correlation values calculated by the preamble correlation module to obtain 16 or 8 correlation peak values, and if the 16 or 8 correlation peak values are greater than a certain threshold, determining that the receiving end searches for an effective preamble 0-bit baseband signal.

In this embodiment, two principles of segment correlation are adopted, and in the direct spread power meter reading system, the Preamble uses the spreading factor 256, so that under the condition that baseband frequency synchronization is not performed at the transmitting end and the receiving end, the OQPSK baseband signal formed by a 256-chip sequence is directly used for correlation processing. In the internet of things, because a high-precision crystal oscillator cannot be basically selected in consideration of equipment cost, the frequency deviation ratio between the transceiver equipment is large. Resulting in an engineering implementation that cannot directly use 256 chips to form the baseband signal for direct correlation calculation.

Firstly, the method comprises the following steps: in this embodiment, since there are at least 8 0 bits as the preamble data, correlation calculation is performed first before and after receiving baseband data, that is, correlation calculation is performed using 256x8 received I/Q sample data and 256x8 received I/Q sample data. If there is a significant correlation peak, it indicates that the receiving end has searched for the chip start and end positions of the two leading 0 bits. As noted in fig. 7 (1).

Secondly, the method comprises the following steps: the receiving end receives 256x 8I/Q data (8 times chip clock rate sampling), and generates 256 chip I/Q baseband data locally, the total length of the baseband data is 256x8, the first segment is divided into 16 segments of local baseband signals, and each segment of the baseband signals has the length of 16x 8. As noted in (2) of fig. 7. For coarse frequency adjustment.

Thirdly, the method comprises the following steps: the receiving end receives 256x 8I/Q data (8 times chip clock rate sampling), generates 256 chip I/Q baseband data locally, the total length of the baseband data is 256x8, and the second segment is divided into 8 segments of local baseband signals, namely, the length of each segment of local baseband signals is 32x 8. As noted in (3) of fig. 7. For fine frequency adjustment.

Fourthly: the frequency adjustment is completed through the three previous processes, in this process, the preamble chip baseband signal with the length of 256 × 8 and the received signal are directly correlated, the correlation peak value is calculated, and the first fuzzy phase adjustment is performed. The same method is used for the second phase adjustment. As indicated at (4) and (5) in fig. 7.

Principle for receiving baseband signal and local baseband signal to carry out correlation calculation in the invention

Firstly, the method comprises the following steps: according to the OQPSK modulation definition of the direct spread power system, 256 chips generated by leading bit 0 generate I/Q two-path data, and for convenience of analysis and processing, the data is expressed in a complex mode, wherein a real part represents an I path, and an imaginary part represents a Q path. The baseband data transmitted by the transmitting end can be expressed as: (a)₁+b₁j),(a₂+b₂j),...,(a₂₅₆+b₂₅₆j) As labeled (1) in fig. 8.

Secondly, the method comprises the following steps: for intuitive analysis, a baseband signal sent by a sending end is expressed in a polar coordinate manner as follows:

in the same way, the locally generated baseband signal is represented as:

as noted in (2) of fig. 8.

Thirdly, the method comprises the following steps: and (3) correlation calculation, namely, recording the correlation as conjugate correlation of the two signals, and then performing accumulation processing, wherein the correlation is expressed by a formula:

as noted in (3) of fig. 8.

Assuming that a preamble baseband sequence received at a receiving end and a locally generated preamble baseband sequence are the same, if the operating frequencies of the transmitting end and the receiving end are the same, psi ═ Φ in an xCorr calculation formula_i-θ_iShould be a fixed value. If the baseband operating frequencies at the two ends of the transceiver are different, the shorter the correlation sequence is, the more stable the value of Δ Ψ is.

Therefore, in the invention, 16 sections of short local baseband signals are used firstly, then 8 sections of long local baseband signals are used, and finally the spread spectrum baseband signals corresponding to the whole bits are used for carrying out correlation calculation.

Fourthly: frequency deviation calculation method

According to the xCORr value obtained by the correlation calculation, be can be obtained by each calculation^jΨWherein the absolute value abs (be) of b^jΨ) Is the correlation peak and Ψ is the correlation phase. The correlation difference Δ Ψ between two adjacent correlation peaks can be considered as the phase offset between the two transmit and receive ends. Δ t represents the time length of two adjacent correlation peaks.

2πΔfΔt＝ΔΨ

Δf＝ΔΨ/(2πΔt)

fifth, the method comprises the following steps: with respect to phase ambiguity (phase synchronization)

The phase ambiguity of the receiving end exists even under the condition that the frequency of the transmitting end and the receiving end are synchronous, the phase ambiguity problem exists, the theory is embodied in that when the correlation calculation is carried out, a fixed psi value exists due to the deviation of the transmitting end and the receiving end, and the phase ambiguity elimination is to multiply an e value on a receiving signal^-jΨ. The result of the correlation of the transmitted and received signals is a real number. It should be noted that only the correlation peak and the phase difference are of interest during the synchronization search and frequency compensation process. In actual engineering, the base bands at the transmitting end and the receiving end cannot be completely synchronized, and phase compensation needs to be performed in real time.

The invention completes the frame leading search process of electric meter reading

Step 1: frame search or frame header search in the power meter reading system is completed at a receiving end, the receiving end continuously receives frame burst data from the transmitting end, the frame burst data passes through a radio frequency front end, down-conversion is carried out, and finally sampling is carried out at 8 times of a chip rate to obtain two paths of I/Q baseband signals. As in step 1 of fig. 9.

Step 2: the local preamble chip is segmented in a first way. Locally leading a 0 bit, generating 256 chips according to the requirements of direct-spread power meter reading specifications, and dividing 256-chip sequence data into 16 segments, wherein each segment is a segmented chip sequence with the length of 16. According to the requirements of direct spread spectrum power meter reading specifications, OQPSK is adopted for modulation, and local baseband data of 16I/Q two channels are formed. Finally, the 16I/Q local baseband data are continuously subjected to 8 times of upsampling processing, and essentially, each I/Q data is repeated 8 times to form 16 sections of local I/Q baseband signals, and the length of each section of local I/Q baseband signal is 16x 8. As shown in step 2 of fig. 9.

And step 3: correlation processing is performed using the 16-segment segmented baseband data signal and the received baseband signal, and 16-segment correlation calculations are calculated each time a sample is received. 16 correlation peaks were obtained. As in step 3 of fig. 9.

Using the calculated 16 correlation values, a frequency deviation of the baseband signal is preliminarily calculated, and then the frequency deviation is used to perform frequency compensation on the received baseband signal.

And 4, step 4: and a second local preamble chip segmentation mode. The local preamble is one 0 bit, 256 chips are generated, and the 256-chip sequence data is divided into 8 segments, each segment being a segmented chip sequence of 32. And according to the requirements of direct sequence spread spectrum power meter reading specifications, OQPSK modulation is adopted to form local baseband data of 8I/Q two channels. And finally, continuously carrying out 8 times of up-sampling processing on the 8I/Q local baseband data, wherein the essence is that each I/Q data is repeated 8 times to form 8 sections of local I/Q baseband signals, and the length of each section of local I/Q baseband signals is 32. As shown in

steps

4 and 5 in fig. 9.

And 5: and after the correlation calculation and the frequency adjustment are carried out on the local baseband signals in the first mode and the second mode in a segmenting mode, finally, the correlation calculation is carried out by adopting the non-segmented baseband signals to obtain a correlation value. After the

steps

3 and 4, the non-segmented baseband signal is correlated to obtain a correlation value, and the phase of the correlation value is used for phase compensation. As shown in

steps

6, 7 and 8 in fig. 9.

The performance of the present invention in an actual direct-spread power meter reading system will be described below using a scenario where different frequency deviations exist at the transmitting end and the receiving end. According to the description of the present embodiment, the correlation results are divided into (a) correlation results are directly performed without segmentation, (b) correlation results are performed in 16 segments, and (c) correlation results are performed in 8 segments.

Firstly, the method comprises the following steps: in a scenario where there is no frequency deviation, as shown in fig. 10. There is good correlation performance without segmentation and with both 16 and 8 segmentation approaches.

Secondly, the method comprises the following steps: in a scenario where there is a frequency deviation of 200Hz, as shown in fig. 11. There is good correlation performance without segmentation and with both 16 and 8 segmentation approaches. The frequency deviation of 200Hz of the baseband signals at the receiving end and the transmitting end does not affect the receiving performance of the receiving end, namely the baseband signals can be normally received without a sampling segmentation mode.

Thirdly, the method comprises the following steps: in a scenario where there is a frequency deviation of 400Hz, as shown in fig. 12. Both 16 and 8 segmentation methods have good correlation performance, but no segmentation direct correlation calculation exists, and no obvious correlation peak exists. The frequency deviation of the baseband signals at the two ends of the receiving and transmitting terminal is 400Hz, and if the preamble segmentation processing is not sampled, the preamble signal cannot be normally searched.

Fourthly: in a scenario where there is a frequency deviation of 2500Hz, as shown in fig. 13. The method adopts the non-segmented preamble to directly carry out correlation calculation without obvious correlation peak values, when 16-segment segmented preambles are adopted to carry out correlation, although the correlation peak values can be identified, interference exists, only 16 correlation peak values are originally needed, 19 correlation peak values are identified, the 19 correlation peak values are still effective, and after the 19 correlation peak values are adopted to carry out frequency adjustment, 8-segment segmentation is carried out again, and 8 better correlation peak values can still be obtained.

From the above simulation, if the present invention is not used, the receiving end can only suppress the baseband frequency deviation of 200Hz, but after the method of the present invention is used, the receiving end can suppress the baseband frequency deviation of 2500 Hz. In this embodiment, even if a low-cost crystal is selected, it is still possible to suppress the baseband frequency deviation existing at both the transmitting and receiving ends by using an algorithm.

Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.

Claims

1. A system for improving initial synchronization performance of a direct sequence spread spectrum system is characterized in that: the device comprises a baseband signal receiving unit, a correlation calculation unit, a preamble segment data correlation peak value searching unit, a frequency deviation calculation unit, a segment chip generating unit of a local one-bit preamble chip, and a local baseband signal generating unit of a segment chip;

the correlation calculation unit is used for performing correlation calculation on the local leading subsection chip baseband data and the received baseband signal, and performing the correlation calculation once when receiving one chip symbol data;

the frequency deviation calculating unit performs frequency estimation using the correlation likelihood value obtained by the correlation calculating unit to obtain the frequency deviation.

2. The system for improving initial synchronization performance of a direct sequence spread spectrum system according to claim 1, wherein: the frequency deviation calculating unit firstly calculates the phase difference between two adjacent correlation peaks according to the determined correlation values, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the frequency deviation of the base band between the receiving end and the transmitting end to adjust the frequency of the base band.

3. A method for improving initial synchronization performance of a direct sequence spread spectrum system is characterized in that: the method comprises the following steps:

and step 3: receiving a baseband signal to form a baseband data stream, wherein the baseband data stream is up-sampled by m times of the baseband signal;

and 4, step 4: respectively carrying out correlation calculation on the received baseband data stream and the local segmented chip sequence modulation signal; when a receiving end receives and moves one sampling data, the receiving end performs correlation calculation with a local segmented chip sequence modulation signal once, and K correlation peak values are obtained each time;

and 5: searching a correlation peak by a receiving end, and if K obvious correlation peak values exist, indicating that the receiving end has searched a preamble baseband signal of one bit 0 in the frame burst;

4. The utility model provides a system for improve direct spread electric power meter reading system initial synchronization performance which characterized in that: the device comprises a power meter reading signal receiving module, a local preamble chip data generating module, a local preamble baseband data generating module, a preamble correlation calculating module, a preamble section data correlation peak value searching module and a frequency synchronization calculating module, wherein the preamble section data correlation peak value searching module and the frequency synchronization calculating module are used for improving the initial synchronization performance of a direct spread power meter reading system;

the direct spread power meter reading system adopts a frame burst mode for transmission, each burst data block consists of an SHR, a PHR and a PSDU, and the burst data block is modulated by adopting an offset quadrature phase shift keying OQPSK modulation mode;

the leading correlation calculation module is used for carrying out correlation calculation on the received baseband digital signals and the segmented leading modulation baseband data, and carrying out correlation calculation once when a baseband digital signal sampling value is received to obtain 16 or 8 correlation values;

5. The system for improving the initial synchronization performance of the direct spread power meter reading system according to claim 4, wherein: the local segmentation principle includes the following:

a receiving end receives 256 multiplied by 8I/Q data, the total length of the baseband data is 256 multiplied by 8, the baseband data is locally generated into 256 chip I/Q baseband data, the baseband data is segmented into 16 sections of local baseband signals for the first time, and the length of each section of baseband signal is 16 multiplied by 8 and is used for coarse frequency adjustment;

and (3) finishing frequency adjustment, directly carrying out correlation calculation on the preamble chip baseband signal with the length of 256x8 and the received signal in the process, calculating a correlation peak value, carrying out first fuzzy phase adjustment, and carrying out second phase adjustment by adopting the same method.

6. The system for improving the initial synchronization performance of the direct spread power meter reading system according to claim 5, wherein: the correlation calculation includes the following:

(a₁+b₁j),(a₂+b₂j),...,(a₂₅₆+b₂₅₆j)

the locally generated baseband signal is represented as:

and (3) correlation calculation, namely, recording the correlation as conjugate correlation of the two signals, and then performing accumulation processing, wherein the correlation is expressed by a formula:

7. the system for improving the initial synchronization performance of the direct spread power meter reading system according to claim 6, wherein: the frequency deviation calculation method includes:

according to the xCORr value obtained by the correlation calculation, be is obtained by each calculation^jΨAbsolute value abs (be)^jΨ) That is, psi is a correlation phase, the correlation difference Δ psi between two adjacent correlation peaks is the phase deviation between the two ends of the transceiver, and Δ t represents the time length of the two adjacent correlation peaks;

2πΔfΔt＝ΔΨ

Δf＝ΔΨ/(2πΔt)

8. The system for improving the initial synchronization performance of the direct spread power meter reading system according to claim 4, wherein: the frame leading search process of the electric meter reading comprises the following steps:

performing correlation processing by using 16 sections of segmented baseband data signals and received baseband signals, and calculating 16 sections of correlation calculation once when receiving a sample value to obtain 16 correlation peak values;

and after the correlation calculation and the frequency adjustment are carried out on the local baseband signals in the first mode or the second mode in a segmenting mode, the correlation calculation is carried out by adopting the baseband signals in non-segmenting mode to obtain a correlation value, and the phase of the correlation value is used for carrying out phase compensation.